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SubjectRe: IO scheduler benchmarking
On Fri, Feb 21, 2003 at 02:51:46AM -0800, William Lee Irwin III wrote:
>> Restricting io in flight doesn't actually repair the issues raised by

On Fri, Feb 21, 2003 at 12:08:07PM +0100, Andrea Arcangeli wrote:
> the amount of I/O that we allow in flight is purerly random, there is no
> point to allow several dozen mbytes of I/O in flight on a 64M machine,
> my patch fixes that and nothing more.

I was arguing against having any preset limit whatsoever.


On Fri, Feb 21, 2003 at 02:51:46AM -0800, William Lee Irwin III wrote:
>> it, but rather avoids them by limiting functionality.

On Fri, Feb 21, 2003 at 12:08:07PM +0100, Andrea Arcangeli wrote:
> If you can show a (throughput) benchmark where you see this limited
> functionalty I'd be very interested.
> Alternatively I can also claim that 2.4 and 2.5 are limiting
> functionalty too by limiting the I/O in flight to some hundred megabytes
> right?

This has nothing to do with benchmarks.

Counterexample: suppose the process generating dirty data is the only
one running. The machine's effective RAM capacity is then limited to
the dirty data limit plus some small constant by this io in flight
limitation.

This functionality is not to be dismissed lightly: changing the /proc/
business is root-only, hence it may not be within the power of a victim
of a poor setting to adjust it.


On Fri, Feb 21, 2003 at 12:08:07PM +0100, Andrea Arcangeli wrote:
> it's like a dma ring buffer size of a soundcard, if you want low latency
> it has to be small, it's as simple as that. It's a tradeoff between
> latency and performance, but the point here is that apparently you gain
> nothing with such an huge amount of I/O in flight. This has nothing to
> do with the number of requests, the requests have to be a lot, or seeks
> won't be reordered aggressively, but when everything merges using all
> the requests is pointless and it only has the effect of locking
> everything in ram, and this screw the write throttling too, because we
> do write throttling on the dirty stuff, not on the locked stuff, and
> this is what elevator-lowlatency address.
> You may argue on the amount of in flight I/O limit I choosen, but really
> the default in mainlines looks overkill to me for generic hardware.

It's not a question of gain but rather immunity to reconfigurations.
Redoing it for all the hardware raises a tuning issue, and in truth
all I've ever wound up doing is turning it off because I've got so
much RAM that various benchmarks could literally be done in-core as a
first pass, then sorted, then sprayed out to disk in block-order. And
a bunch of open benchmarks are basically just in-core spinlock exercise.
(Ignore the fact there was a benchmark mentioned.)

Amortizing seeks and incrementally sorting and so on generally require
large buffers, and if you have the RAM, the kernel should use it.

But more seriously, global io in flight limits are truly worthless, if
anything it should be per-process, but even that's inadequate as it
requires retuning for varying io speeds. Limit enforcement needs to be
(1) localized
(2) self-tuned via block layer feedback

If I understand the code properly, 2.5.x has (2) but not (1).


On Fri, Feb 21, 2003 at 02:51:46AM -0800, William Lee Irwin III wrote:
>> The issue raised here is streaming io competing with processes working
>> within bounded memory. It's unclear to me how 2.5.x mitigates this but
>> the effects are far less drastic there. The "fix" you're suggesting is
>> clamping off the entire machine's io just to contain the working set of

On Fri, Feb 21, 2003 at 12:08:07PM +0100, Andrea Arcangeli wrote:
> show me this claimping off please. take 2.4.21pre4aa3 and trash it
> compared to 2.4.21pre4 with the minimum 32M queue, I'd be very
> interested, if I've a problem I must fix it ASAP, but all the benchmarks
> are in green so far and the behaviour was very bad before these fixes,
> go ahead and show me red and you'll make me a big favour. Either that or
> you're wrong that I'm claimping off anything.
> Just to be clear, this whole thing has nothing to do with the elevator,
> or the CFQ or whatever, it only is related to the worthwhile amount of
> in flight I/O to keep the disk always running.

You named the clamping off yourself. A dozen MB on a 64MB box, 32MB on
2.4.21pre4. Some limit that's a hard upper bound but resettable via a
sysctl or /proc/ or something. Testing 2.4.x-based trees might be a
little painful since I'd have to debug why 2.4.x stopped booting on my
boxen, which would take me a bit far afield from my current hacking.


On Fri, Feb 21, 2003 at 02:51:46AM -0800, William Lee Irwin III wrote:
>> a single process that generates unbounded amounts of dirty data and
>> inadvertently penalizes other processes via page reclaim, where instead
>> it should be forced to fairly wait its turn for memory.

I believe I said something important here. =)

The reason why this _should_ be the case is because processes stealing
from each other is the kind of mutual interference that leads to things
like Mozilla taking ages to swap in because other things were running
for a while and it wasn't and so on.


-- wli
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